A massive MIMO (massive MIMO) system has attracted wide attention in academics and industry in recent years. The theoretical research shows the massive MIMO system can significantly improve spectrum efficiency and energy efficiency simultaneously by a simple linear algorithm such as the zero-forcing algorithm, the minimum mean-square error algorithm and the like. Therefore, the massive MIMO system is likely to be adopted as a key technique in the next-generation communication standard.
However, for example, performance of the massive MIMO system in multi-cell time division multiplexing scenario is limited due to a problem of pilot pollution. Specifically, since the length of a pilot is limited to a coherent length of a channel, the number of orthogonal pilots is limited, and it is inevitable that a pilot is reused among different cells. In this case, pilot signals transmitted by the users, located in different cells, which use a same pilot sequence or pilot sequences which are incomplete orthogonal may be received by a same base station. However, the base station can not distinguish the pilot signals effectively, and thus channel estimation at the base station is disturbed. In a case that the base station adopts the disturbed channel estimation to detect uplink data, not only data transmitted by a user in the present cell is received, but also data transmitted from a user in another cell is received, which results in inter-cell interferences in the uplink. In a case that the base station adopts the disturbed channel estimation to generate a pre-coding matrix and transmit downlink data, the downlink data can be received not only by the user in the present cell, but also by the user in the another cell, which results in inter-cell interferences in the downlink interference.
The theoretical research shows that, although both the spectrum efficiency and energy efficiency of the massive MIMO system can be improved significantly, and an influence of the noise and a channel estimation error on the performance of the massive MIMO system is becoming smaller with the increase of the number of antennas in the base station, the inter-cell interferences caused by the pilot pollution can not be eliminated, which becomes one of limiting factors for the performance of the massive MIMO system.
The existing methods for alleviating the pilot pollution are difficult to be applied considering the current technical condition, and therefore, the pilot pollution is still one of the serious problems of the massive MIMO system in the actual application. In addition, the pilot pollution becomes worse with the continuous increase of the number of the users within the cell.